1. Technical Field
The present invention relates to the field of projecting laser light or other electromagnetic radiation. More specifically, the present invention relates to systems, methods, and optical devices utilized in the conversion of a laser point source to produce a laser reference plane. Such substantially planar and omnidirectional laser projection finds, among many uses, surveying and leveling applications in the construction industry.
2. Prior and Related Art
The technology of generating omnidirectional and planar projections from laser sources is relevant to many sensory and fiduciary applications. Machine-vision inspection systems may require illumination over a given plane. Illumination planes may also be used as datum or reference markers in augmented reality systems. Without limiting the scope of the present invention, the following discussion is provided in the context of the specific problem of generating accurate visual reference planes for use in the construction industry. The requirements of this specific application are illustrative of the difficulties involved in the generation of a planar projection from a laser source and the technical advantages of the invention.
A survey of prior art reveals five types of approaches to converting a laser point source into a planar projection. These approaches can be categorized as including: (a) rotating elements, (b) conical reflector, (c) cone lens, (d) cylindrical lens, and (e) fiber-optic bundle.
The first approach rotates a laser source or optical components (such as prisms or mirrors) to mechanically trace an omnidirectional (i.e., 360 degrees) projection plane. Prior art such as U.S. Pat. Nos. 4,062,634 (Rando) and 4,830,489 (Cain) disclose embodiments that are vulnerable to mechanical vibrations, gyroscopic forces, misalignment, and degradation of component surfaces. The embodiments that involve physical motion not only require high manufacturing and assembly costs with precision components and extensive alignments but also are susceptible to the harsh environmental and field conditions common in the construction industry.
The second approach eliminates the complications resulting from physical motion by utilizing one or more conical reflectors. The incident beam, directed towards the apex of the conical reflector along its axis of rotational symmetry, is reflected by the lateral surface into an omnidirectional planar projection. U.S. Pat. No. 4,111,564 (Trice) teaches a variation of the inventive concept. The “distance-variable planar diffusion” embodiment includes a configuration of two reflecting cones mutually opposed and axially aligned. The incident beam passes through a hollow central portion of the first cone and is reflected by the lateral surface of the second cone. Thereafter, the reflection is received by the lateral surface of the first cone and reflected into a planar projection. The planarity of the resulting projection can theoretically be achieved by the appropriate curvature of the lateral surfaces and distance between them. In practice, embodiments utilizing one or more conical reflectors require precise alignment of the incidence angle of the beam with respect to the axes of rotational symmetry of the reflectors. Over long distances, a slight misalignment will result in significant deviation of the projection from the desired plane. The specific teachings of U.S. Pat. No. 5,335,244 (Duncan) that address these difficulties in alignment are ill-suited for the rugged demands of the construction trades.
The third approach utilizes cone lenses to convert a laser beam into a planar projection. Concave and convex cone lenses are commonly manufactured as purely refractive optical elements. U.S. Pat. No. 6,754,012 B2 (Terauchi) and U.S. Pat. No. 4,111,564 (Trice) teach the utilization of concave cone lenses with lateral surfaces coated with reflective film to redirect an incident laser beam through the interior of the lens body to be emitted from the circumferential bounding surface of the lens. A planar projection that is orthogonal to the optical axis can be achieved by a calculated balance of the apex angle of the concavity, the refractive index of the lens material, and the angle of the bounding surface in accordance with Snell's law of refraction. U.S. Pat. No. 6,754,012 B2 (Terauchi) teaches a variation of this third approach in which the incident beam is redirected at the lateral surface by total internal reflection thereby eliminating the need for a reflective coating. For embodiments that include cone lenses, like those which utilize conical reflectors, the planarity of the resulting projection is extremely sensitive to the alignment of the incidence angle of the beam with respect to the optical axis of the cone lens. Furthermore, since cone lenses introduce thermal dependencies, as governed by the thermoptic coefficient of the lens material, the planarity and orthogonality of the resulting projection varies as the index of refraction changes with temperature. Over long distances, small changes in temperature would result in substantial deviations of the resulting projection.
The fourth approach utilizes various convex and concave curvatures of cylindrical lenses to disperse an incident beam into a projected fan of laser light. While also characterized by the exigencies of its own particular optical alignment and by similar susceptibilities to temperature variations, cylindrical lenses are limited by a more prohibitive shortcoming in that the projected fan or plane is not omnidirectional. Thus, cylindrical lenses find limited relevance to construction applications that require more expansive reference planes.
The last approach, as taught by U.S. Pat. No. 5,898,809 (Taboada), takes advantage of the “localization of light” phenomenon in a conventional fiber-optic bundle given an orthogonally-incident laser beam. There are several disadvantages of this approach for the specific application of generating laser reference planes. Even though individual fibers may be consistently uniform, the bundling process may result in variations and irregularities that limit the accuracy of the projected plane over long distances. Compensation for such deviations using a calibration process would be difficult due to the complexity and number of material interfaces. These interfaces would also argue for conversion inefficiencies that may adversely impact system design. Furthermore, similar to the prior art discussed above, this approach does not teach how to compensate for thermal effects due the wide variation in ambient temperature in the construction field.
The five types of approaches to converting a laser point source into a planar projection, as described above, do not teach or suggest utilizing ring lenses, prism-rings, or spherical lenses. Ring lenses as found in U.S. Pat. No. 1,774,842 (Peters) and App. Pub. No. U.S. 2007/0205961 (Black) are relevant to applications in which the accuracy of the resulting image or projection is not critical (such as lighthouse illumination and RF transmission). Prism-ring lenses, as taught by prior art such as U.S. Pat. No. 6,016,223 (Suzuki) and U.S. Pat. No. 5,191,479 (Tsuchida), consist of a plurality of refractive prism surfaces that are used for beam shaping in applications which require specific projections (such as double-Bessel beam) or for applications in which the accuracy of the resulting image or projection is not critical (such as Fresnel lenses). Prism-ring lenses, as taught by U.S. Pat. No. 6,616,305 (Simon), may also have a plurality of reflecting prisms for specific lighting applications. However, none of the prior art for prism-ring lenses teaches a plurality of reflective and/or refractive concentric prisms that redirect the incident beam in a serial or compound manner in which the emitted beam from one prism is received by another prism.